Continuous presses of the aforedescribed type are widely used to press mats of wood chips, wood fibers, sawdust and the like, usually admixed with a thermally activatable or thermosetting binder, into a continuous strand which can be then subdivided into pressed board for use in a wide variety of structural and other applications. Depending on the pressing characteristics, the pressed board can have narrow thickness tolerances, high quality surfaces and surfaces which have a high degree of smoothness. Oriented Strand Board (OSB) has wood-based strands or chips with lengths ranging between 30 and 150 mm and width/length ratios of 1:5 to 1:10 and thicknesses of 0.25 to 1.5 mm. These elongated chips are deposited with a preferred orientation, usually in the longitudinal direction, and give rise to especially high strength and high quality boards.
It is common, with such systems, to press the mat after it has been pretreated with a heating fluid.
Substantially all of the boards described are exclusively smooth-surfaced members.
There are, however, also on the market wood-based pressed boards which have structured surfaces on one or both of its broad sides. Structured surfaces are advantageous because they increase the frictional coefficient of the surface, increase the ability of glue to anchor to the surface or otherwise provide advantages in the use of the product. The structuring can be in the form of alternating rises and depressions, also referred to as bumps and grooves. Up to now, continuous presses of the type described could not, to the best of our knowledge, be utilized effectively in the formation of structured surfaces for high quality pressed board.
To produce structured surfaces in wood-based pressed board, a cyclically operating system was used, usually involving platen presses and, most commonly, multilevel platen presses. The mats were charged into the various levels of the press upon a tray or other support and the press platens were provided with structuring complementary to that desired in the pressed board or a steel wire screen was placed upon the charging tray or interpositioned between the platen and the mat to impress the pattern of that screen in the board as it was pressed.
However, such cyclically operating systems present problems with respect to the charging and emptying of the press, problems with handling the pressed board and charge trays subsequent to the pressing operation and problems with removal of the wire mesh or sieve-like patterning members.